With the rapid spread of information-related and communication apparatuses such as personal computers, video cameras, and mobile phones in recent years, the development of batteries utilized as power supplies for the apparatuses has been regarded as important. The development of high-output and high-capacity batteries for electric or hybrid vehicles has also been pursued in the automobile industry and the like. Among the various batteries, lithium batteries currently attract attention from the viewpoint of having high energy densities.
In currently commercially available lithium batteries, electrolytic solutions including flammable organic solvents are used, and therefore it is preferable to attach safety devices that inhibit temperature from increasing in the event of a short circuit and to improve structures and materials for preventing a short circuit. In contrast, all-solid-state lithium batteries in which solid electrolyte layers are used instead of electrolytic solutions are considered to allow the simplification of safety devices and to be excellent in production cost and productivity because any flammable organic solvent is not used in the batteries. However, the energy densities of the all-solid-state lithium batteries are lower than those of liquid-based batteries under present circumstances.
Sulfide solid electrolyte materials are known as solid electrolyte materials used in all-solid-state lithium batteries. For example, a Li—P—S-based sulfide-based solid electrolyte has been reported (see, for example, Non Patent Literatures 1 to 5). However, the previously reported crystalline sulfides have had an ion conductivity of around 10−7 to 10−4 Scm−2 and have been incapable of sufficiently realizing the higher energy densities of all-solid-state lithium batteries.
Non Patent Literature 6 has reported Li10GeP2S12 (hereinafter may be referred to as “LGPS-based sulfide solid electrolyte”, “LGPS”, or the like) exhibiting a high ion conductivity of 12×10−3 Scm−1 comparable to the ion conductivity of an electrolytic solution. However, previously proposed Li—Ge—P—S-based sulfide solid electrolytes such as Li10GeP2S12 (LGPS) reductively decompose at a potential of around 0.25 V based on lithium (vs Li/Li+, hereinafter the same applies) and therefore preferably have further enhanced electrochemical stability for use of the Li—Ge—P—S-based sulfide solid electrolytes in lithium batteries.